As a follow on to the Acoustic Thermometry of Ocean Climate (ATOC) project
regular acoustic transmissions across the North Pacific Ocean have continued over
the past six years. ATOC was originally funded by DARPA and the this work
continues as part of the North Pacific Acoustic Laboratory (NPAL) project
funded by the Office of Naval Research (ONR). The transmitted signals are received
on U.S. Navy SOSUS and other listening arrays of hydrophones in the northeast Pacific. The
travel times of the coded acoustic signals are a precise measure of the average temperature
of the ocean. This measurement is valuable because the ocean is highly variable, so
measurements at a single point, for example by a thermometer, are dominated by local
temperature fluctuations which are usually unrelated to climate change.

Acoustic thermometry was accepted as part of the global ocean observing system during
the OceanObs'99 Conference. The
summary report [Koblinsky, C. and N. Smith, eds., Observing the Oceans in the 21st
Century (Melbourne: GODAE Project Office and Bureau of Meteorology, 2000)] states (p. 28):

Acoustic tomographyThe Conference was provided with a detailed account of the potential for acoustic tomography to
make sustained contributions to the observing system
[available from the link at left]
and of the potential value of acoustic
thermometry. Till this point, it was a technique that had not figured prominently in the
planning for either GOOS/GCOS or for CLIVAR. In terms of the scientific approach to sampling
the full depth of the ocean, globally, acoustic tomography offers some attraction since it
provides long-path, integral (low wave-number) measurements of thermal variations to complement
those from Argo and satellite altimetry.

The Conference concluded that acoustic tomography did represent a potentially valuable approach
and that, initially, it should be implemented in the Arctic and at specific locations such as
the Straits of Gibraltar. The Conference also encouraged an exploratory implementation in the
North Atlantic in the presence of substantial profiling floats to test the complementarity
and/or redundancy between tomography and other measurements. The Pacific Ocean plans also
include the use of acoustic tomography in the North Pacific.

Changes in the climate state of the North Pacific are obvious in the acoustic time
series. The central North Pacific warmed considerably during 1999-2001 and the annual
cycle in ocean temperature was relatively weak. But in early 2003, the central Pacific
Ocean rather suddenly cooled and developed a strong annual cycle. Similar changes were
observed using TOPEX/POSEIDON satellite altimetry. Satellite altimetry is a somewhat
less precise measure of ocean temperature (as a column of water warms, sea surface
height rises), but altimetry gives the larger geographic context to the changes that
are occurring on the acoustic paths. These climate changes are probably normal variations
in the Earth's climate system. After 6 years, no obvious sign of "global warming" is apparent.

The time series apparently spans a cycle of the Pacific Decadal Oscillation (PDO - a phenomena
that dominates Pacific Ocean climate variations), though the changes observed do not appear
to be strictly related to the PDO. The climate state of the North Pacific Ocean has been
shown to have a profound effect on the weather of North America.

This project is a contribution to CLIVAR (CLIVAR is derived from "CLImate VARiability and
Predictablity")- an international research program to study climate changes and the impacts
of changing climate on health, weather, agriculture, etc.

The ocean is highly variable, with changes occurring over yearly to decadal to century (and
beyond) time scales. The difficulty in detecting these changes, distinguishing the signals
occurring at many time and length scales, and distinguishing between anthropogenic and
natural climate changes, are reasons why the long-range acoustic technique is an important
tool for climate observation.